The present application claims priority to Japanese Patent Application No. 2000-064501, filed in the Japanese Patent Office on Mar. 9, 2000, and Japanese Patent Application No. 2001-51421, filed in the Japanese Patent Office on Feb. 27, 2001. The contents of those applications are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile and similar image forming apparatuses, and more particularly to a fixing device for an image forming apparatus.
2. Discussion of the Background
An image forming apparatus, for example, a copying machine, a printer and a facsimile, includes an image bearing member. An electrostatic latent image corresponding to image information is formed on the surface of the image bearing member and is visualized with toner.
A toner image is transferred to a sheet, and the toner image on the sheet is fixed thereon by heat and pressure in a fixing device.
For example, a fixing device employing a heat roller method includes a fusing roller and a pressure roller. The fusing roller and the pressure roller contact each other across a sheet conveying path. A nip part, i.e., a contact part, is formed between the fusing roller and the pressure roller. When the sheet passes through the nip part, a toner image on the sheet is fixed thereon by applying heat and pressure to the toner image.
In the above-described fixing device, the fusing roller is arranged at a position where the fusing roller directly contacts a toner image on a sheet. Therefore, fused toner on a sheet is likely to adhere to the circumferential surface of the fusing roller.
When toner does not completely permeate a sheet, the sheet may cling to the circumferential surface of the fusing roller together with the toner due to the viscosity of the toner. When the sheet clings to the fusing roller, the sheet is conveyed along the curvature of the fusing roller, thereby deviating the sheet from a sheet conveying path. This type of sheet jam results in a sheet staying in the fixing device.
The fixing device further includes a separation pick in the vicinity of the fusing roller so as to separate a sheet moving along the curvature of the fusing roller from the fusing roller. The separation pick cooperates with a sheet guide member arranged opposite to the separation pick and directs the sheet separated from the circumferential surface of the fusing roller to the sheet conveying path.
Because the separation pick has a sharp tip that abuts the circumferential surface of the fusing roller, there is no gap for a sheet to go into between the separation pick and the fusing roller. Hence, the sheet is properly separated from the fusing roller by the separation pick.
However, because the tip of the separation pick is constantly in contact with the circumferential surface of the fusing roller, over the course of time, the tip of the separation pick may be worn, and toner adhered to the surface of the fusing roller may be collected around the tip of the separation pick. As a result, the tip of the separation pick may be separated from the circumferential surface of the fusing roller, thereby causing a sheet to go into a gap formed between the separation pick and the fusing roller. Consequently, a sheet is likely to be jammed between the separation pick and the fusing roller.
A background image forming apparatus has a space around a sheet conveying path in a fixing device which can be opened to remove jammed sheets.
For example, Japanese Laid-open Patent Publication No. 9-274421 describes an image forming apparatus including a fixing device wherein the fixing device is pulled out to outside of a housing of the image forming apparatus by opening an opening/closing door formed as a part of the housing of the image forming apparatus so as to remove jammed sheets from the fixing device.
Japanese Laid-open Patent Publication No. 5-289570 describes an image forming apparatus including a fixing device wherein the fixing device includes an upper unit and a lower unit. In that image forming apparatus, a space around a sheet conveying path can be opened for removing jammed sheets by rotating the upper unit having an upper fixing roller away from the lower unit.
In a fixing device, steam is generated from a sheet because water included in the sheet evaporates under heat applied by a fusing roller. When the steam leaks out from the fixing device, condensation of the steam remaining in an image forming apparatus may occur.
Particularly, when the steam remaining in the image forming apparatus is cooled, condensation may occur in the image forming apparatus and water droplet may attach to a surface of an image bearing member. The water droplet on the surface of the image bearing member typically causes the photosensitive property of the image bearing member to change or a toner image on the surface of the image bearing member to spread. These result in deterioration of image quality.
According to one aspect of the present invention, a fixing device for fixing a toner image on a sheet in an image forming apparatus, includes a frame, a fusing roller rotatably provided in the frame, a pressure roller rotatably provided in the frame and configured to press against the fusing roller, a separating member configured to separate the sheet passed between the fusing roller and the pressure roller from the fusing roller, a sheet guide member configured to guide the sheet separated by the separating member and rotatably supported by the frame, and a shield member configured to prevent air from flowing from a space between the frame and the sheet guide member at a position where the sheet guide member is supported by the frame.
According to another aspect of the present invention, a method of preventing air from flowing out from a fixing device in an image forming apparatus, includes accommodating a fusing roller and a pressure roller in a frame of the fixing device, providing a separating member configured to separate a sheet passed between the fusing roller and the pressure roller from the fusing roller, and a sheet guide member configured to guide the sheet separated by the separating member, the sheet guide member being rotatably supported by the frame, and providing a shield member configured to prevent air from flowing from a space between the frame and the sheet guide member at a position where the sheet guide member is supported by the frame.